Treatment of metallic ferrites



Unite States Patent 3,337,918 TREATMENT Git METALLiC FERRITES Marvin D. Moore, liomewoo-d, and Robert K. Birlts, Elmhurst, liL, assignors to Chicago Copper 8; Chemical fiompany, liiue island, 11., a corporation of Illinois No Drawing. Filed Nov. 13, 1964, Ser. No. 411,106 Claims. (1. 23-51) This invention relates to the preparation of magnetically hard metal ferrites and to permanent magnets made therefrom as well as to methods of producing the same. More particularly, this invention relates to the production of barium, calcium, strontium and lead ferrites which are useful as magnetic materials of high coercive strengths.

Magnetic varieties of these ferrites can be represented by the chemical formula MO.6Fe O wherein M is a divalent metallic ion. These ferrites have a hexagonal crystal structure of the magnetoplumbite type. They are ferromagnetic with a preferred direction of magnetization parallel to the hexagonal axis such that the crystallites will orient themselves in a magnetic field so that the hexagonal axis of the crystallites will be parallel with the magnetic lines of force.

Metallic ferrites which are magnetically hard are used in forming permanent magnet strips which are used in magnetic door closures. Such magnetic strips are generally prepared from a mixture of powdered metal ferrite and a synthetic resin. In such magnetic strips, it is highly desirable to have magnetic ferrites with improved magnetic strength and increased coercive force. With ferrites of improved magnetic properties there is a reduction in the size of the magnetic Strips required for magnetic door closures with a corresponding reduction in the cost of construction and assembly.

The magnetic ferrites are important in industry in the production of permanent magnets. They are usually prepared by firing a mixture of the component oxides or carbonates in an atmosphere of nitrogen, oxygen or air at 1800-2500" F. They are often ground and pressed after the sfirst firing and refired. They are mechanically hard and can be strong or brittle. Barium ferrite, for example, is used in the form of a sintered fine powder and is a good permanent magnetic material. For the foregoing reasons, metallic ferrites are of great commercial importance in the production of permanent magnetic products.

It is an object of this invention to provide improved lead and alkaline earth metal ferrites, such as barium, calcium and strontium ferrites. It is another object to produce metallic ferrites of improved magnetic properties. A further object of this invention is to provide a method for treating metallic ferrites with acid to improve the magnetic properties thereof. A further object is to provide a method for producing magnetically hard metallic ferrites of superior coercive strengths than are available by heretofore known methods. These and other objects are apparent from and are achieved in accordance with the following disclosure.

Broadly, the present invention relates to the treatment of metallic ferrites with aqueous acid to form superior metallic ferrites which have higher and improved residual magnetic induction, intrinsic coercive force, and maximum energy product than the same materials without such acid treatment.

The metal M of the metallic ferrites represented by the formula given above can be an alkaline earth metal such as barium, strontium and calcium, or it can be a Group IV-A metal such as lead.

In the treatment of metallic ferrites with aqueous acid for improvement of magnetic properties, we have found that aqueous solutions having a pH of 4.1 or less are "ice satisfactory. It is possible to operate at lower pH values in the range of 1 or less but this is generally unnecessary because below pH 1 a point of diminishing returns is reached where little additional improvement can be realized even over long periods of treatment. Consequently, the preferred pH range is from 1.0 to 4.1. Any of the readily available acidic materials can be used which will form aqueous solutions having a pH of 4.1 or less. For example, 5% acetic acid or 2.5% hydrochloric acid will give pH values in the operative range. Other acids such as surfuric, phosphoric, hydrobromic, formic, oxalic, toluenesulfonic and the like are also operative. pH values as high as 5 are suitable although the improvement in magnetic properties of the metallic ferrites requires longer periods of time at higher pHs. The stronger the aqueous acid solution or the more severe the treatment, the greater the resulting improvement in the magnetic properties of the metallic ferrite will be.

The amount of acid is relatively small with respect to the amount of metallic ferrite being treated. On a stoichiometric basis substantially less than 1 equivalent of acid per equivalent mole of metallic ferrite is used. Generally about mole equivalent of acid per equivalent mole of metallic ferrite is satisfactory and even lower quantities of acid are operative.

The acid treatment of the metallic ferrites can be conducted at room temperature (20 C.) to the boiling point of the aqueous acid (100110 C.). At higher temperatures the treatment proceeds more rapidly whereas at lower temperature longer times are necessary to attain the same improvement in the magnetic properties of the metallic ferrite. At boiling temperature, treatments for periods of 5 to 10 minutes achieve probably of the improvement in the magnetic properties of the metallic ferrites and longer treatment periods only serve to attain the final 10% of improvement. At room temperature it is usually desirable to treat the metallic ferrite with aqueous acid for at least an hour or two to attain maximum improvement.

The magnetic properties of the metallic ferrite powders produced by the procedure of this application can be compared to the magnetic properties of prior art metallic ferrites by the following procedure. 30.0 grams of the metallic ferrite are compounded with 1.2 grams of resin binder and 0.15 gram of zinc stearate (die lubricant) and pressed into a cylinder in a die 1.1 inches in diameter under a total force of 40,000 pounds. The resulting metallic ferrite cylinder is then magnetized to saturation in a direction parallel to the axis of the cylinder. The lifting force, which is a measure of the magnetic remanence and a function of the coercive force of the resulting magnet, is determined by measuring the upward attraction that one of the end surfaces of the cylinder exerts on a piece of soft iron attached to a balance pan. This lifting force has been found to vary in a definite linear relationship to the height of the cylinder. Therefore, in all tests the lifting force of the specimen is corrected to a standard cylinder height which has been taken as 0.58 inch. It is seen by a review of the lifting force of the ferrites treated in accordance with the procedure of this application, as described in Examples 1 through 4, inclusive, that the lifting force is considerably greater than that of the ferrite produced by the prior art procedure.

The invention is described in further detail by means of the following examples which are provided for the purposes of illustration only. It will be understood by those skilled in the art that various modifications in equivalent materials, relative amounts of materials and operating conditions can be made within the disclosure of this application without departing from the invention as herein described.

Example 1 40 grams of commercially available barium ferrite was boiled for 30 minutes in 150 cc. of aqueous acetic acid. The metallic ferrite was then removed by filtration, washed and dried. The resulting ferrite was tested and evaluated by the procedure described herein. It was found to have a lifting force of 240 grams when 30 grams of material was compressed into a cylinder 1.1 inch in diameter and 0.58 inch thick. The untreated commercially available barium ferrite, when compressed into the same size cylinder, had a lifting force of only 228 grams.

Example 2 Another sample of commercial barium ferrite was ball milled in 50 cc. of 15% aqueous acetic acid. The barium ferrite was then removed and boiled for 30 minutes in 150 cc. of 5% aqueous acetic acid, the barium ferrite was removed by filtration, washed and dried. When 30 grams of this material was compressed into a cylinder 1.1 inch in diameter and 0.58 inch thick this ferrite was found to have a lifting force of 239.6 grams compared to 233.2 grams for the same material after similar treatments with water containing no acid. This represents a 2.75% improvement in lifting force after acid treatment.

Example 3 The barium ferrite of Example 2 was ball milled in 2.5% hydrochloric acid (50 cc.) and then boiled in 150 cc. of 2.5 hydrochloric acid for 30 minutes. The barium ferrite was then removed by filtration, washed and dried. A cylinder 1.1 inches in diameter and 0.5-8 inch thick containing 30 grams of this material had a lifting force of 246 grams. This represents a 5.5% improvement in lifting force due to the acid treatment.

Example 4 Another specimen of commercially available barium ferrite was molded into a test cylinder 1.1 inches in diameter and 0.58 inch thick containing 30 grams of material and evaluated in the manner described above. It was found to have a lifting force of 234.8 grams. A specimen. of the same barium ferrite was boiled for 30 minutes in 150 cc. of 5% hydrochloric acid, filtered, washed, dried and evaluated. This material was found to have a lifting force of 256.3 grams.

We claim:

1. A method of increasing the residual magnetic induction of a metallic ferrite of the formula MO.6Fe O wherein M represents barium, strontium or lead which comprises treating said metallic ferrite with an aqueous acid solution having a pH not greater than 5 at a temperature in the range from 20 to C.

2. The method of claim 1 wherein the pH of the acid solution is below about 4.1.

3. The method of claim 2 wherein the metallic ferrite has the formula BaO.6Fe O and the acid is hydrochloric acid.

4. The method of claim 2 wherein the metallic ferrite has the formula SrO.6Fe;O and the acid is hydrochloric acid.

5. The method of claim 2 wherein the metallic ferrite has the formula PbO.6Fe O and the acid is hydrochloric acid.

References Cited UNITED STATES PATENTS MHJTON WEISSMAN, Primary Examiner.

HERBERT T. CARTER, OSCAR R. VERTIZ,

Examiners. 

1. A METHOD OF INCREASING THE RESIDUAL MAGNETIC INDUCTION OF A METALLIC FERRITE OF THE FORMULA MO.6FE2O3 WHEREIN M REPRESENTS BARIUM, STRONTIUM OR LEAD WHICH COMPRISES TREATING SAID METALLIC FERRITE WITH AN AQUEOUS ACID SOLUTION HAVING A PH NOT GREATER THAN 5 AT A TEMPERATURE IN THE RANGE FROM 20* TO 110*C. 